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One tiny brain cell is all it takes to restore voluntary movement of paralysed muscles, according to US scientists.

The finding, which appears in the journal Nature, may lead to treatments for people paralysed with spinal cord or other injuries, they say.

The system essentially provides an artificial route for brain signals to reach paralysed muscles, replacing a natural pathway that may have been disrupted by injury.

While other teams have developed complicated systems that look for brain signals that control movement in specific body parts, Dr Chet Moritz and colleagues at the University of Washington wanted to see if the brain could teach itself to use the computerised system.

There are some 100 billion neurons in the human brain and the study suggests an unsuspected degree of flexibility in the kinds of tasks they can perform.

"Nearly every neuron we tested could be used to control this type of stimulation," says Moritz.

He says the system would be intended for use in individuals who are paralysed from the neck down, but "we are several decades away from this being a clinical application".

Spinal cord injuries cripple hundreds of thousands of people worldwide every year, rendering the simplest of actions frustratingly difficult or simply impossible.

Those afflicted with the most severe form of paralysis, known as lock-in syndrome, are fully conscious prisoners inside a body that no longer responds to commands.

While the brain activity that would normally result in a voluntary movement is still present, the instructions simply don't reach the muscles.

Re-routing singals

Earlier experiments enabling monkeys to manipulate prosthetic devices or computer cursors using only electrical impulses coming from the brain were based on a fundamentally different premise, according to the new study.

"They tried to read the mind of the monkey and figure out what he was planning to do," a technique that required massive computing power, says Moritz.

For their study, Moritz and his team connected electrodes to individual neurons inside the motor cortex of the monkey's brain and recorded the electrical activity.

These signals were then routed in real-time to a computer and from there through a stimulator to another set of electrodes attached directly to wrist muscles that had been artificially blocked further up the arm along the normal neural pathway.

Because little processing power is needed, the computer is the size of a cell phone and can be attached to the animal's body.

"Our approach is to recreate the raw connectivity between single neurons in the brain and muscles and let the monkey's nervous system learn how to use that connectivity," says Moritz.

The monkey had already mastered a simple video game, grasping targets shown on a video screen with a control device manipulated by a single hand.

"But once he was paralysed, the only way to move his wrist was to change the activity of individual neurons in his brain."

On average it took about 10 minutes for the monkeys to "train" the neuron well enough to play the video game again.

"The brain can very rapidly learn to control new cells and use them to generate movements," says co-author Dr Eberhard Fetz.

To avoid infections, the system would have to become fully implantable so that no wires passed through the skin.

Electrodes would need to be made more stable so that they could record the activity of neurons over a period of years, rather than weeks.